This application claims priority from European Patent Application No. 04106436.1 filed Dec. 9, 2004, the entire disclosure of which is incorporated herein by reference
The present invention concerns a method for driving a vibrating device for a portable object. The vibrating device comprises a moving mass and a coil of annular shape electro-magnetically coupled to the moving mass, in order to make it oscillate. The vibrating device can be used in particular as a silent alarm or to indicate a telephone call.
There exist several embodiments of vibrating devices capable of fulfilling the function in particular of a silent alarm in order to be fitted to small sized portable objects, such as portable telephones, traditional organizers, pagers or wristwatches. At least one coil of these vibrating devices can be electrically actuated to activate a mass in order to create a low frequency vibration which can be felt by the person wearing such an object.
Generally, the frequency of the electrical signals applied to the coil is adjusted to correspond to the natural mechanical oscillation frequency of the moving mass of the vibrating device. In this way, maximal vibration amplitude can be obtained for a minimum of supplied electrical energy. The vibration of the device can be controlled by specific programming of the portable object so as to warn its user of a specific event, for example an alarm time, a telephone call or suchlike.
Patent document WO 02/46847, which discloses a method for driving such a vibrating device, can be cited in this regard. The vibrating device comprises a moving mass having a permanent magnet and a coil electro-magnetically coupled to the moving mass to make it oscillate. In order to make the mass oscillate, a drive circuit has to supply the vibrating device coil with rectangular drive voltage pulses of alternating polarity and determined length after the vibrating device start phase. The amplitude of the drive pulses approximately corresponds to the battery voltage electrically powering the drive circuit.
Between each drive voltage pulse, the coil is disconnected, i.e. it is placed in a high impedance state. In this state, the coil supplies an induced voltage due to the movement of the permanent magnet of the oscillating mass. A resonant frequency measurement is carried out at each zero crossing of the induced voltage in the drive circuit in order to adjust the period of the rectangular drive pulses supplied to the coil.
One drawback of such a drive method is that at each disconnection of the coil placed in the high impedance state, overvoltages, whose time constant is dependent upon the features of the coil, can be observed. These overvoltages can damage the electronic drive or supply circuit. Moreover, with these overvoltages, before the frequency measurement, a significant idle period has to be observed, which can be of the order of several hundreds of microseconds so as not to detect inadvertent zero crossings. This idle period, which has to be observed, limits the oscillation frequency to a low value. It is therefore necessary to filter the overvoltages by suitable means, either at the input of an amplifier comparator of the circuit, or at the output of the comparator. This involves providing the drive circuit with additional electronic components with the function of driving the oscillations of the moving mass, which complicates the manufacture of said circuit.
Another drawback of the drive method of WO 02/46847 is that the drive voltage pulses are made up of a fundamental frequency f0 and harmonic frequencies f1, f2, which create power losses and stray forces, which oppose the active driving force of the oscillating mass. Consequently, higher electric consumption is observed. In the case of a determined battery voltage, the amplitude of the fundamental frequency signal relative to the rectangular drive pulses is at a voltage level that can be a third less than the battery voltage, and cannot thus be adapted to a higher value.